1. Technical Field
The present invention relates to starters which have a pinion tube spline-fitted on an output shaft and are configured to shift the pinion tube relative to the output shaft in a direction away from a motor and thereby bring a pinion supported on a non-motor-side end portion of the pinion tube into mesh with a ring gear of an engine.
2. Description of Related Art
There is disclosed, for example in Japanese Patent Application Publication No. 2006-177168, a starter that has a cantilever structure.
Specifically, as shown in FIG. 3, the starter includes: an output shaft 100 configured to be driven by a motor (not shown); a pinion tube 120 fitted on the output shaft 100 via a pair of sliding bearings 110; a one-way roller clutch 130 configured to transmit rotation of the output shaft 100 to the pinion tube 120; a pinion 140 that is straight-spline-fitted on a non-motor-side end portion (i.e., a left end portion in FIG. 3) of the pinion tube 120; and a housing 160 that supports the pinion tube 120 via a ball bearing 150 axially positioned between the clutch 130 and the pinion 140. Further, the starter is configured so that with operation of an electromagnetic switch (not shown), the pinion tube 120 and the clutch 130 are together shifted relative to the output shaft 100 in the axial direction away from the motor (i.e., in the leftward direction in FIG. 3), thereby bringing the pinion 140 fitted on the pinion tube 120 into mesh with a ring gear (not shown) of an engine.
With the above starter, however, when the engine has been completely started and thus the pinion 140 comes to be rotated by the engine, the clutch 130 will enter an overrun state where the clutch 130 inhibits torque transmission from the pinion tube 120 (or from the engine side) to the output shaft 100 (or to the motor side). Consequently, the rotational speed of the pinion 140 and the pinion tube 120, which are together rotated by the engine, will become remarkably higher than the rotational speed of the output shaft 100 that is driven by the motor. Therefore, it is necessary to interpose the bearings 110 between the pinion tube 120 and the output shaft 100. In addition, the bearings 110 are press-fitted to the inner periphery of the pinion tube 120, and the output shaft 100 is inserted inside the bearings 110 so as to be rotatable relative to the bearings 110.
Moreover, in recent years, the number of motor vehicles, which are equipped with an Idling Stop System (ISS), has increased to meet the demands of improving fuel economy and reducing exhaust gases. An ISS is generally designed to stop injection of fuel into the engine and thereby automatically stop the engine when the vehicle makes a brief stop for, by way of example, waiting for a traffic light to change or traffic congestion. Therefore, for a starter used in a motor vehicle that is equipped with an ISS, the number of times the starter starts the engine of the vehicle is dramatically increased in comparison with a starter used in a motor vehicle without an ISS. Accordingly, it is required to secure high durability (or long service lives) of starters used in those motor vehicles which are equipped with an ISS.
However, when the starter disclosed in Japanese Patent Application Publication No. 2006-177168 is used in a motor vehicle equipped with an ISS, wear of the bearings 110, which are interposed between the output shaft 100 and the pinion tube 120, will be accelerated by the dramatic increase in the number of times the starter starts the engine of the vehicle. Consequently, relative inclination between the output shaft 100 and the pinion tube 120 will become large. In addition, wear of the bearings 110 occurs when the pinion 140 is rotated by the engine with the clutch 130 in its overrun state so that the bearings 110 are subjected to high-speed relative rotation between the output shaft 100 and the pinion tube 120 and also subjected to high load.
Moreover, with increase in the relative inclination between the output shaft 100 and the pinion tube 120, contact pressure at a spline engagement portion will be increased; at the spline engagement portion, male splines formed on an outer surface of the output shaft 100 engage with female splines formed on an inner surface of an outer of the clutch 130. Consequently, adhesion between the tooth surfaces of the male and female splines may occur, thereby making it difficult to secure high durability of the starter.
In addition, the female splines may be inclined relative to the male splines due to radial and circumferential clearances provided therebetween. In particular, when an ignition switch (or a starter switch) of the vehicle is again turned on after being turned on once and then turned off without completely starting the engine, the pinion 140, which is kept rotating by the inertial rotation of the starter motor, will be again brought into mesh with the ring gear of the engine. Consequently, the rotation of the pinion 140 will be suddenly stopped, inducing an excessive mechanical shock at the spline engagement portion and thereby causing relative inclination between the male and female splines. As a result, with the relative inclination between the male and female splines, an excessive contact pressure will be generated between the male and female splines due to local contact therebetween, thereby causing adhesion between the tooth surfaces of the male and female splines.